Ceramics with controlled porosity find wide applications as catalytic surfaces and supports, adsorbents, chromatographic materials, filters, light weight martials, and thermal and acoustic insulators. In catalytic applications the macropores facilitate material transport to the nanoporous internal regions where reactions can take place. Macroporous silica could be of substantial use as insulating layers in integrated circuits. The low dielectric constant of this material lowers the capacity of the chips which makes them faster. Furthermore, ceramics with regular arrays of pores have unique optical properties such as optical filters which have strong wavelength dependent reflectivity and transmission. They are also candidates for photonic band gap materials. See Yablonovitch, E. J. Opt. soc. Am. B 10, 2830295 (1993); and Joannopoulos, J. D., Meade, R. D. & Winn, J. N. Photonic Crystals: Molding the Flow of Light, 1-137 (Princeton University Press, Princeton, 1995).
Porous materials are commonly categorized according their (average) pore size. Microporous materials have pore diameters .ltoreq.2 nm, mesoporous ones have pores in the range of about 2-50 nm, and macroporous materials contain pores .gtoreq.50 nm. See IUPAC Manual of Symbols and Terminology, Appendix 2, Part 1, Colloid and Surface Chemistry, Pure Appl. Chem. 31, 578 (1972). An extension of these materials to the mesoporous regime is provided by the class of materials known as MCM-41. These mesostructures have channel diameters of 2-10 nm and thus are the extension of zeolites, with pore diameters &lt;2 nm. However, control over the size, shape, and ordering of pores larger than 10 nm has remained a challenge. See J. C. Jansen, M. Stocker, H. G. Karge, and J. Weitkamp (editors), "Advanced Zeolite Science and Applications", Studies in Surface Science, Vol. 85, (Elsevier, Amsterdam, 1994); C. T. Kresge et al., Nature 359, 710 (1992); and J. S. Beck et al., J. Am. Chem. Soc. 114, 10834 (1992); J. S. Beck et al., U.S. Pat. No. 5,108,725 (1992). These aluminosilicates are prepared through a liquid crystal mechanism in which a sol-gel process takes place in the interstitial regions of an ordered surfactant phase formed by self-assembly of rodlike micelles acting as templates. See Krauss, T., Song, Y. P., Thomas, S., Wilkinson, C. D .W. & DelaRue, R. M. Electron. Lett. 30, 1444-1446 (1994). This results in cubically or hexagonally ordered pores, the size of which can be controlled by varying the surfactant and the amount of solubilized additives. Pore sizes are in the range of 1-10 nm. This class of materials has been extended with a number of transition metal oxides. See D. M. Antonielli and J. Y. Ying, Angew. Chem. Intl. Ed. Engl. 34(18), 2014 (1995); 35(4), 426 (1996).
For larger pore sizes, most notably in the macroporous regime, no method is known to produce ceramics containing periodic pores. It has been possible, however, to produce pores with a well-defined (spherical) shape, namely by the foaming method and by the hollow sphere sintering method. The first method uses a colloidal sol or a powder slurry containing a surfactant which is foamed with a gas and then gelled by a sol-gel reaction. See T. Fujiu, G. T. Messing, and W. Huebner, J. Am. Ceram. Soc. 73(1), 85 (1990); M. Wu, T. Fijiu, and G. L. Messing, J. Non-Cryst. Solids 121, 407 (1990). In the second method, hollow microspheres are first blown out of a molten oxide (e.g. glass) which are then sintered together or incorporated into a ceramic matrix. For a review, see R. L. Downs, M. A. Ebner, and W. J. Milner, In: L. C. Klein (Ed.), "Sol-Gel Technology for Thin Films, Fibers, Performs, Electronics, and Specialty Shapes", (Noyes Publications, Park Ridge, N.J., 1988), pp 330-381. In both cases large pores are produced in the range of roughly 10 to 1000 .mu.m. Pore sizes are generally broadly distributed and do not allow for assembly into regular lattices.
Finally, some colloidal microspheres (most notably silica) can be made sufficiently monodisperse such that they form regular (cubic ) arrays under the right circumstances. These packings can then be sintered to a ceramic. See T. J. Garino and H. K. Bown, J. Am. Cera. Soc.70, C311, C315 (1987); A. P. Philipse, J. Mater. Sci. Lett. 8(12), 1371 (1989). Porosities of these packings are low, however (around 26%), and not easily controlled.